The present invention relates to an optical mount. The alignment status of large tools and jigs used in airplane assembly factories are measured and reset at periodic intervals. Standard optical tooling, consisting of jig transits, optical levels, alignment telescopes, lasers, etc., are typically used in the re-alignment process. With standard optical tooling, it is possible to align jigs and tools to a fairly high degree of accuracy, typically to within a few thousandths of an inch over distances of 20 or 30 feet (limited by the angular resolution capability of the optical tooling, and the finesse of the human operator). However, alignment checks are usually only possible when a tool is empty, because one or more tooling fiducials are typically obscured when a part is loaded on the tool. This leads to the possibility of part rework due to undetected misalignment conditions which may occur during part build-up.
LAST (Laser Alignment System for Tooling, U.S. Pat. No. 4,889,425; issued Dec. 26, 1989 and assigned to The Boeing Company) was developed to provide for continuous monitoring of tool alignment to 0.001" over distances of 150 to 200 feet. The phototype LAST was comprised of a low-power CW laser, optical area-array detectors and associated electronics, and a 3-dimensional laser gage beam network created by means of parallel-plate optical beam splitters rigidly attached to the testbed jig at strategic sites along the beam path.
The prototype LAST was shown to be capable of detecting displacements in the testbed jig as small as 0.0004" over distances of 150 feet, which implies an instantaneous angular resolution capability of 1/20th of an arc second. However, this resolution capability was not actually realized on the testbed jig because of time-varying local disturbances in the boresight direction of the laser and in the angular orientation of the beam splitters. Thus, although very minute disturbances in the alignment of the testbed jig were detectable by LAST, the exact nature of the disturbances was difficult or impossible to discern.
In order to realize the full benefit of LAST and other such systems which utilize laser beams as gages, it is essential that the beamsplitters located at critical branching points in the beam path, along with the laser itself, be isolated from externally induced attitude disturbances. It is clear that the level of isolation must be extremely high in order to prevent unwanted angular disturbances of the gage beam from masking actual jig motions of the order of 0.0004" over distances of 150 feet (i.e., on the order of 1/20th arc second). If the laser gage beam is successfully isolated from the jig structure, the deflection data provided by the optical detectors may then be used to prescribe a series of corrective actions to bring the tool or jig to an undistorted state, thereby driving the observed deflections to zero.
One approach for monitoring (but not maintaining) the dimensional integrity of the laser gage beam involves the use of electronic tilt sensors, which can be used to monitor changes in the attitude of the beamsplitters and in the laser itself. Tilt sensors are available which are capable of providing reliable tilt data for angular changes of less than 1 arc second. This approach requires the use of a single tilt sensor for monitoring the pitch attitude of the laser, and a pair of tilt sensors (for pitch and roll) attached to each beamsplitter located at each major node in the laser gage beam. This approach is obviously quite complex, and may be quite costly, depending on the number of instrumented nodes along the beam path.
In the patent literature, Funazaki et.al., U.S. Pat. No. 4,854,704 has shown the use of a laser diode light source focused on a pentaprism by a set of lenses (one of which is damped) in an optical automatic leveling apparatus. The Pentaprism in Funazaki et.al. is rotatably mounted in contrast to the present invention in which it is the suspended element. Damping means in aforementioned U.S. Pat. No. 4,854,704 is magnetic in contrast to fluid damping for the prism in the present invention. Funazaki et.al. utilizes a three wire single stage suspension system in contrast to the Present invention which utilizes a two stage four wire system that essentially will gimbal from the external attachment cap.
Fluid damping per se is known in the prior art as exemplified by U.S. Pat. No. 2,843,001 issued Jul. 15, 1958 to Werner where a filament suspended pendulum utilizes a "dash pot" (or fluid damping) means in an optical tilt correcting device.